From what I've seen of Taco Bell's "Mexican Pizza", it's really just a bastardized tostada, not something you bake in an oven like a pizza. You can buy ready-to-use tostadas if you live in a Mexican neighborhood, but they're easy enough to make at home. Simply fry a wheat flour or masa tortilla in oil at around 375 degrees until it crisps and darkens slightly, and dry on paper towels. It's then ready to top as wish, even with pizza toppings, if you like.